The United States on Tuesday called on Sudan to investigate the killings of three UN peacekeepers in an ambush in the troubled western Darfur region.

Three Senegalese troops died, and a fourth was wounded, when a water convoy they were escorting was attacked on Sunday on a road from the town of El Geneina to the UN mission in Darfur (UNAMID) headquarters in West Darfur.

The United States is trying to persuade a key Syrian opposition group to drop its refusal to join planned peace talks, saying its participation is essential, a U.S. official said Tuesday.

The Syrian National Council, which is the biggest bloc within the Syrian opposition coalition, said at the weekend it would not attend the talks planned for next month and would quit the umbrella group if it does.

A man set himself on fire near the U.S. Congress on Friday, police said, one day after a woman was shot dead by law enforcement after a high-speed chase nearby.

"It does appear that a man sustained self-inflicted burns," Metropolitan Police Department spokeswoman Gwendolyn Crump said.

Republicans launched a furious counter-attack on the White House as the U.S. government shutdown dragged deep into a fourth day, but no fresh path to ending the crisis was offered.

With lawmakers heading into the weekend, Congress appeared no closer to resolving the impasse, even as a far more challenging fiscal hurdle -- the need to raise the U.S. debt ceiling or suffer a catastrophic credit default -- was barely 12 days away.

U.S. police shot a female motorist dead after she led them on a high-speed chase from near the White House through the streets of downtown Washington to the Capitol building.

Lawmakers and tourists scattered and ducked for cover Thursday as shots rang out during a police operation to halt the woman, who was driving with a one-year-old girl in her black sports car.

The United States on Sunday condemned the killings of two Israeli soldiers in separate incidents on the West Bank.

"Such violence and terror are unacceptable," State Department spokeswoman Jen Psaki said.

The mother of the defense contractor who gunned down 12 people at the Washington Navy Yard apologized Wednesday for her son's rampage and said she did not understand his actions.

"To the families of the victims, I am so, so very sorry that this has happened. My heart is broken," Cathleen Alexis said in a statement that she read to the media at her home in Brooklyn, New York.

U.S. Defense Secretary Chuck Hagel acknowledged on Wednesday that authorities missed some "red flags" that might have prevented the deadly mass shooting at the Washington Navy Yard.

Hagel made the admission as he announced the details of a sweeping review of security at all military bases in the aftermath of Monday's attack that left 13 dead, including the gunman, at a naval installation in the heart of Washington.

The Pentagon said Tuesday it would review security at U.S. bases worldwide after a defense contractor gunned down 12 people in Washington, as investigators worked to uncover the shooter's motive.

A day after the shooting, officials were struggling to answer how and why 34-year-old Aaron Alexis, a former sailor with a history of disciplinary problems and brushes with the law, had been granted a security clearance.

U.N. leader Ban Ki-moon wants Sudan's President Omar al-Bashir to answer war crimes charges, a spokesman said Tuesday as controversy mounted over Bashir's bid to attend a U.N. summit next week.

Bashir, who is wanted by the International Criminal Court for genocide in Darfur, has embarrassed the U.S. government and United Nations by seeking a visa to enter the United States to attend the annual U.N. General Assembly next week.